A rear door of an automobile includes a damper stay that reduces the operation force required to open and close a rear door and hold the door at a fully open position. The damper stay includes a first end, rotatably coupled to the rear door, and a second end, rotatably coupled to a vehicle body. Patent document 1 describes an example of an opening/closing assist device in which an elongated hole is formed at a portion of the rear door where the first end of the damper stay is coupled so that a first end (door side coupling point) of the damper stay is movable relative to the rear door along the elongated hole. An assist moment based on an elastic force of the damper stay is controlled by the movement of the first end of the damper stay along the elongated hole to reduce the operation force required to open and close the rear door.